This invention relates to a method for purifying an organic solution containing lactams. More specifically, it relates to a method for effectively separating and purifying an effective ingredient contained in an organic solution by extracting "an organic solution mainly containing one or more kinds of lactam components" which also contains an anionic surface active substance as an impurity with water.
The method of the present invention can be suitably used when separating .epsilon.-caprolactam and .omega.-dodecanelactam by extracting with water "an organic solution containing an anionic surface active substance as an impurity" which is obtained by extracting a mixture of .epsilon.-caprolactam and .omega.-dodecanelactam with an organic solvent.
As the method for producing .epsilon.-caprolactam and .omega.-dodecanelactam simultaneously, known method is as follows. That is, a mixture of cyclohexanone and cyclododecanone is simultaneously oximated, the resulting mixture containing the produced cyclohexanone oxime or a salt thereof and cyclododecanone oxime or a salt thereof is subjected to Beckmann rearrangement in the presence of sulfuric acid or fuming sulfuric acid. Then, a lactam mixture is obtained by neutralizing the above mixture with ammonia gas or aqueous ammonia, and the lactam mixture is extracted with an organic solvent which is immiscible with water or a lactam oil layer separated from an ammonium sulfate aqueous layer by phase separation of the lactam mixture is extracted with an organic solvent which is immiscible with water to obtain an extract containing lactam components. Further, the extract is reverse extracted with water to transfer .epsilon.-caprolactam into the aqueous layer whereas .omega.-dodecanelactam remains in the organic solvent layer (so-called "colactamization method"), which method is described, for example, Japanese Patent Publications No. 7254/1971 and No. 10168/1971).
In the simultaneous preparation method of .epsilon.-caprolactam and .omega.-dodecanelactam according to the "colactamization method", the extract containing the above lactam components has heretofore been extracted with water as such. However, in said extracted solution, anionic surface active substances such as alkyl sulfate derived from a straight or branched alkyl alcohol contained in cyclododecanone used as a starting material are present so that, when effecting extraction with water, the organic solvent layer and the aqueous layer are difficult to separate in many cases due to the formation of an emulsion. According to this reason, an extracting tower with a large tower diameter is required. Further, an organic material is contained in the aqueous layer and the aqueous layer becomes turbid whereby causing bad effects on the quality of .epsilon.-caprolactam, and water-soluble impurities are also contained in the organic solvent layer to cause negative effects on the quality of .omega.-dodecanelactam.